A conventional technique is to isolate lubricant from a leakage of blow-by gas from a combustion chamber and to return the blow-by gas without lubricant to the intake side (such as an intake manifold) of the engine (see, for example, patent document 1 and patent document 2).
Another conventional technique is to remove lubricant using chambers each incorporating a plurality of oil trap materials (filtration nets) (patent document 1). Still another conventional technique is to remove lubricant using a spiral member having a variable spiral pitch (patent document 2).